Long-life Inorganic Electrochromic Device Based on WO3 and PB Films with Fast Switching Respond

Complementary electrochromic devices were fabricated by using tungsten trioxide (WO3) film as working electrode, Prussian blue (PB) film as counter electrode and 0.1 M lithium perchlorate in propylene carbonate as electrolyte. The XRD result presents that WO3 and PB films are amorphous, which is favourable for a fast ion intercalation-deintercalation process. The device was tested under potentiostatic control using ±1.3 V steps, and changed its color between blue and colorless. It has a fast-switching respond (about 1.5 s for colouring and 2 s for bleaching) and could withstand 100,000 cycles with little change in its optical contrast.


Introduction
Electrochromic devices (ECDs) are electrochemical cells that can reversibly change their optical properties for an applied potential due to an electrochemical redox reaction.More and more research has been carried out on ECDs over the past few decades for its potential commercial applications in smart windows, mirrors and eyewears [1][2][3][4][5][6].The tunable light transmittance resulting from the color change of electrochromic (EC) films is much desired in smart windows, which not only improves the aesthetics of traditional windows but also contributes to save energy consumption by reducing heating and cooling loads of building interiors [7].
An ECD is composed of three parts: an EC layer, an ion storage layer and electrolyte.The EC layer is an active layer which triggers the color change in respond to an applied voltage.Organic molecules, organic polymers and inorganic materials are well known as three different kinds of electrochromic materials.Inorganic electrochromic material based on transition metal oxide has good transmittance modulation in the wide wavelength range and weather resistance.WO3 as one of the most widely studied and industrially used electrochromic materials is widely known for its excellent transmittance modulation in the visible and near-infrared light range [8][9][10][11].PB, also known as iron (Ⅲ) hexacyanoferrate, is a conventional blue colored pigment which has been a promising material widely used in biosensors [12,13], electrocatalysis [14,15] and batteries [16,17].PB exhibits several unique merits: excellent electrochemical property, reversibility and non-toxicity in the EC process.It plays an important role of switching its color from Prussian blue (oxidized state) to Prussian white (reduced state) and also serves as an ion storage layer to work with WO3 film as a pair [18][19][20].
In an ECD, the properties of electrochromic and ion storage layers are the primary factor in determining the properties of device.In addition, especially for an ECD based on the inorganic electrochromic material, the charge capacity match between working and counter electrodes are the important factor affecting the performance of device.Compared with WO3 film, the PB film has fast respond speed, but lower charge capacity.A thicker PB film has relatively large charge density.In our study, a multi-step electrodeposition method was used to prepare the thicker PB films with more uniform and condensed surface.WO3 and PB films were prepared using wet-chemical.The obtained films were amorphous, which was favourable for ion mobility in the films.The ECD showed a high durability and stable transmittance modulation.

Chemical materials
All solvent and chemicals were of analytical grade and used without further purification.ITO-coated glass was purchased from Wuhu Token Science Co., Ltd.(China).Lithium perchlorate was purchased from Aladdin Chemistry Co., Ltd.(China).Other reagents were obtained from Sinopharm Chemical Reagent Co., Ltd.(China).All aqueous solutions were freshly prepared with de-ionized water.

Preparation of EC films
Electrodeposition method was used to prepare WO3 film.The electrodeposited process was carried out in three-electrodes cell (CHI650D, chinstruments, Shanghai, China).ITO glass, Pt plate and Ag wire were dipped into the precursor solution and respectively used as working, counter and reference electrodes.The electrodeposition voltage is -0.5 V, and the electrodeposition time is adjustable to obtain different film thickness.The precursor solution was prepared as the following: At room temperature, 60 ml 30% hydrogen peroxide (H2O2) and 6g W metal powder react violently to obtain colorless colloidal solution, which is filtrated to obtain milky solution.After the ultrasonication, the milky solution was refluxed at different temperature in order.60 ml ethanol was added in and refluxed at 50℃ for 12h to obtain transparent orange peroxopolytungstic acid (PPTA) solution.The PPTA solution was used as the precursor solution for preparing WO3 film.More details could be found in our previous research [21].
The precursor solution for PB deposition was composed of 50 mM K3Fe (CN)6, 50 mM FeCl3 and 50 mM HCl in deionized water.Chronoamperometry was applied to electrodeposit PB thin films on ITO with a potential of -0.05 V for 2 s.The thickness of PB film was controlled by the times of repeating electrodeposition.
Electrochemical properties of films were measured by a half-cell test method.Ag wire and Pt sheet were selected as the reference electrode and counter electrode.Optical properties were recorded using a UV-Vis/NIR spectrophotometer (V-670, JASCO International Co., Ltd.Tokyo, Japan) with air as a reference.The morphology of WO3 films was examined using field-emission scanning electron microscopy (JSM-6700F, JEOL, Tokyo, Japan).The crystal structures were investigated by X-ray Powder diffraction (TTR-Ⅲ, Rigaku, Tokyo, Japan).

Fabrication of EC device and cycle test
The ECD was assembled like a sandwich structure with the WO3 film as a working electrode, 0.1M LiClO4 in PC as nonaqueous electrolyte and the PB film as an ion storage layer.After laminating working and counter electrode together, solution type electrolyte was injected into the gap between two electrodes.UV cured glue was used as a hermetic barrier to seal the device.The device was fabricated in the colored state.Cycle tests of the EC device were conducted by using the potential step method in the range of -1.3 V and 1.3 V. Chronocoulometry was done at ±1.3 V for 10 s to analyze the charge storage capacitance.Optical properties of the device were measured by a UV-Vis/NIR spectrophotometer.

Morphology and structures of as-prepared films
In order to improve the adhesion at the WO3-ITO interface, the WO3 film was calcined in air.The morphology of WO3 films can be influenced greatly by the annealing temperature.Smooth and flat surface was formed at a slow heating speed to 110℃ as shown in Figure 1 (a).However, several cracks on the surface were found if the wet WO3 film was placed at a constant annealing temperature of 110℃ for 10 h as shown in Figure 1 (b).The formation of such cracks is mainly attributed to the thermal stress generated by uncontrolled evaporation rate of water and solvent molecules inside the film.In order to obtain better surface morphology, the following annealing condition was adopted: first at 60℃ for 1 h, at 110℃ for 3 h and the heating speed was about 3℃/min.When depositing a thicker PB film by electrodeposition, larger PB particles were formed after continuous electrodeposition, which could reduce the film transparency and the adhesion to substrate.In order to prepare thick and homogeneous PB films, repeating short-time electrodeposition was used.Figure 2 shows the surface morphology of PB films with thickness 410 nm prepared by continuous (c, d) and multi-step (a, b) electrodeposition.Compared with one-step deposition, the PB film prepared by multi-step electrodeposition consists of small particles on its surface, and the surface is more uniform and condensed.The long-lasting deposition could cause larger PB particle on the surface, which could reduce the transmittance of film and the adhesion to the ITO substrate.So, we adopted the multi-step deposition method to prepare PB film with different thickness.Figure 3 shows the X-ray diffraction (XRD) patterns of the as-prepared WO3 film and PB film.The phases of films were identified by XRD, using Cu Kα1 (λ= 0.151841 nm) radiation between 3ºand 60º.Both films show the amorphous structure without any peaks representing crystalline structure.The electrochromic performance of films is closely related to its crystallinity.Amorphous films have a faster response time and higher coloration efficiency compared to crystalline one.

Coloration efficiency of WO3 film
The coloration efficiency of WO3 film with different thickness is calculated to assess the optimum thickness.The equation formula is as follows: Where Tb and Tc are the transmittance of film in bleached and colored states at a given wavelength.A represents the effective measured area, and q is the inserted charge capacity into A.
The wavelength of 580 nm was picked out as the testing wavelength.Figure 4 (a) presents the coloration efficiency as a function of film thickness.The efficiency achieves a maximum in the range of 500-700 nm.The coloration efficiency of WO3 film was 35 cm 2 /C.As shown in Figure 4 (b), the transmittance of WO3/ITO glass reaches over 85% in the visible region.The transmittance modulation got up to 80% at the wavelength range from 600 nm to 700 nm.

Charge density of PB film
Figure 5 presents the charge capacity of PB films with different thicknesses, and the applied potential voltage is ±0.8 V.The PB film changes from colourless to blue between reduced and oxidized states.As expected, with the increase of film thickness, the charge density increases while color switching response slows down as shown in Figure 5 (a) and (b).To take both response time and charge density into account, PB film with a thickness of about 100 nm is selected as the counter electrode of our ECDs.

EC properties of WO3/LiClO4+PC/PB device
Fast switching and stable ECDs were obtained in our research.The EC properties were tested using chronocoulometry.The potential was switched between -1.3 V and 1.3 V to measure the cyclic stability.Figure 6 (a) shows that the bleaching time is always shorter than coloration time.During the first few cycles, coloration occurs in 2s while bleaching occurs in about 1.5 s.Ion intercalation-deintercalation maintains a balance during this process.Initial charge density of the ECD is about 10 mC/cm 2 .As shown in Figure 6 (b), the charge density remains stable before 70,000 cycle times, leading to a stable color change.However, it has a clear tendency to decrease from 70,000 to 100,000 cycle times.Longer coloration and bleaching time are needed to produce similar transmittance after 70 K cycles.As shown in Figure 6 (c), a transmittance modulation of 45% -50% in the visible region was observed for the ECD.After 100 K cycle times, the transmittance slightly rises in the colored state while the bleaching transmittance is fairly consistent in the whole process.It becomes more difficult for the device to switch from fully colored state to bleached state after long time cycle test, mainly due to the gradual failure of PB film.Figure 6 (d) presented the transmittance modulation of device was measured at 580 nm and 950 nm wavelength.The values were about 50% and 45%, respectively.The colored device had low transmittance (＜1%) at wavelength 950 nm.
Figure 7 also illustrated that the current of device decreased from 70,000 to 100,000 cycles.The ion mobility became slower and difficult.The current in the ion deintercalation process was larger than that in the intercalation, which was consistent with the previous conclusion.The bleaching time was shorter than coloration time.Figure 8 shows the results of cycle test using thinner PB film (about 60 nm) as counter electrode.As the cycle test proceeds to over 150 K times, there was a slight decrease in both transmittance and charge capacity of the device.The transmittance modulation presents a significant reduction as the PB film switches from blue to light blue, different from the initial colorless.It reveals that the lifetime of ECD would be improved using thinner PB films in sacrifice of the transmittance modulation.

Conclusions
Complementary ECDs were fabricated using WO3 films as working electrode, Prussian blue as counter electrode and 0.1M LiClO4 in PC as electrolyte.WO3 films were prepared by a sol-gel route and they present the amorphous state, which was favourable for a fast ion intercalation-deintercalation process.PB films were obtained via a layer-by-layer electrodepositing method.The optimum thicknesses of WO3 (500-700 nm) and PB (100 nm) films were found considering the overall performance of the EC properties.Our fabricated ECD has fast switching response (about 1.5 s for bleaching and 2 s for coloring) and stable optical performance.The transmittance modulation remains unchanged after 100,000 cycles with a slightly longer time for applied potential.The lifetime of ECDs would be improved using thinner PB films, sacrificing the transmittance modulation.

Figure 1 .
Figure 1.SEM images of amorphous WO3 films under different annealing condition.

Figure 2
Figure 2 shows the surface morphology of PB (c-d) films with thickness 410 nm, respectively prepared by multi-step (c, d) and continuous (a, b) electrodeposition.

Figure 3 .
Figure 3. XRD patterns of as prepared WO3 film (a) and PB film (b).

Figure 4 .
Figure 4.The coloration efficiency (a) and optical property (b) of WO3 films.

Figure 5 .
Figure 5.The charge density (a) and the charge capacity at the 15th second (b) of PB films with different thicknesses.

Figure 7 .
Figure 7. Current of device during the cycling test, the cycling test was conducted at ±1.3V for 3 s.